The present invention is directed to a composition and a process for developing electrostatic latent images. More specifically, the present invention is directed to a developer composition (and a process for using said composition) which results in reduced image defects such as slipping of the developed image on the photoreceptor, blurred edges of solid area images, washed out fine lines, fuzzy halftones and the like. One embodiment of the present invention is directed to a developer composition which consists essentially of a toner consisting essentially of a resin, a colorant, a charge control agent, and colloidal silica external additive particles and a carrier consisting essentially of a core, an optional coating on the core, and an external additive selected from the group consisting of metal salts, metal salts of fatty acids, linear polymeric alcohols comprising a fully saturated hydrocarbon backbone with at least about 80 percent of the polymeric chains terminated at one chain end with a hydroxyl group, polyethylene waxes with a molecular weight of from about 300 to about 2,000, polypropylene waxes with a molecular weight of from about 300 to about 3,000, and mixtures thereof.
Developer compositions wherein the toner contains external additives such as silica particles or metal salts of fatty acids are known. For example, U.S. Pat. No. 4,948,686 (Koch et al.), the disclosure of which is totally incorporated herein by reference, discloses developers suitable for the formation of colored images wherein the toners contain colloidal silica external additives, fatty acid metal salt external additives, and in some instances external additives consisting of a linear polymeric alcohol comprising a fully saturated hydrocarbon backbone with at least about 80 percent of the polymeric chains terminated at one chain end with a hydroxyl group. The disclosed developers are suitable for the formation of two-colored images in a single development pass, wherein the imaging member is charged to three different levels of potential, the black toner is used to develop one level of potential, the colored toner is used to develop another level of potential, and the third level of potential remains undeveloped as background areas. Imaging processes of this type are also disclosed in, for example, U.S. Pat. No. 4,078,929, the disclosure of which is totally incorporated herein by reference. Also of interest with respect to the tri-level process for generating images is U.S. Pat. No. 4,686,163, the disclosure of which is totally incorporated herein by reference.
Typically, toners that contain colloidal silica and metal salts of fatty acids as external additives contain the silica to improve the flow properties of the toner particles, assure adequate triboelectric charging, enhance admix times (the time required for uncharged toner to become fully charged when mixed with a developer containing a carrier and a charged toner of the same composition as the uncharged toner), and improve temporal stability of the charging properties of the developer. The metal salts of fatty acids typically are added for the purpose of maintaining sufficient conductivity in the toner to assure development with a conductive "magnetic brush" development system.
One difficulty that can be encountered when employing developers wherein the toner contains both silica and a fatty acid metal salt as external additives is deposition of the fatty acid metal salt on the imaging member. Eventually, deposition of the fatty acid metal salt on the imaging member can cause the imaging member to become sufficiently slippery that the tangential forces of the flowing developer striking the developed image overcome the product of the force on the toner as a result of its charge and the electric field in which it resides and the coefficient of friction between the toner and the imaging member, resulting in irregular sliding of the image on the imaging member surface (an image defect sometimes referred to as "scoop" or "slip"). Subsequent to deposition of the fatty acid metal salt on the imaging member, the silica additive can become embedded in the fatty acid metal salt deposited on the imaging member. At high relative humidity, the silica thus deposited can absorb sufficient quantities of water to become conductive, resulting in lateral conductivity of the latent electrostatic image on the imaging member, causing copy quality defects such as blurry edges of solid area images, washed out fine lines, wider and lower density than expected lines, fuzzy half tones, and the like.
The developer and process of the present invention enables the reduction or elimination of this difficulty while retaining the advantages of external additives such as fatty acid metal salts and linear polymeric alcohols.
U.S. Pat. No. 4,073,980 (Westdale et al.) discloses carrier particles for use in an electrophotographic process which are prepared by applying a mixture of a perfluoro acid and molybdenum disulfide to the surface of the carrier particle. The resulting carriers have a very thin film deposited on the surface thereof and are long lived and abrasion resistant.
U.S. Pat. No. 4,331,756 (Mayer et al.), the disclosure of which is totally incorporated herein by reference, discloses electrophotographic developer compositions containing carrier, toner, and special purpose additives such as flow promoters, dry lubricants, and the like. The developers are prepared by coating carrier particles with a coating selected so that the triboelectric relationship between the surface of the carrier and the surface of the additive is substantially zero.
U.S. Pat. No. 4,847,176 (Sano et al.) discloses a binder type carrier comprising at least magnetic particles and a binder resin having an acid value of 50 mg KOH/g or less and a hydroxyl value of 50 mg KOH/g or less, in which a product of the acid value by the hydroxyl value is within the range of from 1 to 600, which gives a high specific volume resistance of equal to or more than 10.sup.13 ohm-cm and an excellent humidity resistance.
U.S. Pat. No. 4,921,771 (Tomono et al.) discloses a toner for developing electrostatic images which comprises a coloring agent, a styrene homopolymer or copolymer with a vinyl monomer or monomers, and polypropylene having a number average molecular weight of about 3000 to 4000 in an amount between about 0.02 and 40 parts by weight per 100 parts by weight of the styrene homopolymer or copolymer.
U.S. Pat. No. 4,920,023 (Koch et al.), the disclosure of which is totally incorporated herein by reference, discloses a process for the preparation of stable developer compositions which comprises treating coated carrier particles with metal salts or metal salts of fatty acids and thereafter admixing these particles with a colored toner composition containing metal salts or metal salts of fatty acids and comprising resin particles and colored pigment particles, wherein the salts are present in an amount of from about 0.01 to about 1 percent by weight.
U.S. Pat. No. 4,960,665 (Elder et al.), the disclosure of which is totally incorporated herein by reference, discloses a toner comprising resin particles, and a component with a sponge or non-flake like morphology selected from the group consisting of metal salts, metal salts of fatty acids, and mixtures thereof.
U.S. Pat. No. 4,614,165, the disclosure of which is totally incorporated herein by reference, discloses a process which comprises transporting a developer material comprising at least carrier granules and toner particles from a housing storing a supply thereof in a chamber to the surface of a photoconductive member having an electrostatic latent image recorded thereon, and discharging toner particles and carrier granules into the chamber of the housing with the carrier granules being added to the chamber of the housing so that the usable life of the developer material is at least equal to the usable life of the imaging machine containing the photoconductive member and with the ratio of toner particles to carrier granules by weight being supplied to the chamber of the housing being substantially greater than the ratio of toner particles to carrier granules by weight in the chamber of the housing.
While known compositions and processes are suitable for their intended purposes, a need remains for developer compositions that generate images of high quality. In addition, there is a need for developer compositions containing both silica external additives and fatty acid metal salt, linear alcohol, or wax external additives wherein deposition of the fatty acid metal salt, linear alcohol, or wax external additive on the imaging member is reduced. Further, a need exists for developer compositions with good flow properties, adequate triboelectric charging, rapid admix times, temporal stability, and adequate conductivity for conductive magnetic brush development processes. Additionally, there is a need for developer compositions that reduce slipping of the developed image on the imaging member. There is also a need for developer compositions that contain both silica external additives and fatty acid metal salt, linear alcohol, or wax external additives and that do not result in lateral conductivity of the latent image on the imaging member.